Add short section about how to run the tests locally to the readme.
Add short section about how to run the tests locally to the readme.

fabrique/django-sortedm2m,gradel/django-sortedm2m,gradel/django-sortedm2m,gradel/django-sortedm2m,fabrique/django-sortedm2m,gregmuellegger/django-sortedm2m,gregmuellegger/django-sortedm2m,gregmuellegger/django-sortedm2m,fabrique/django-sortedm2m